The present invention pertains to the traffic control systems and devices with means for controlling the speed and lane usage of traffic through informational messages directed to the conflicting drivers, with particular emphasis on the control of traffic where the drivers' vision is limited by the roadway construction or terrain features.
Roadways require free flow of traffic to avoid congestion and the resulting increase in driving times. Common impediments to traffic flow are junctions between two or more roadways. Safer and more efficient roadways are possible if traffic can be guided through these junctions.
One of the solutions to the traffic problems at junctions is the use of ramps to merge traffic onto or off of one roadway to another. Often the surrounding terrain does not permit one driver to directly view existing or merging vehicles when approaching a junction. Little time is left for the driver to react and avoid a collision. Even an abrupt avoidance manuever can cause an accident between adjacent or following vehicles who would have otherwise been unaffected by the merging vehicle.
A number of traffic devices have been developed, but are inadequate to control traffic safely and efficiently under all circumstances, especially when a direct view between merging vehicles is prohibited. For example, U.S. Pat. No. 3,304,539 granted to Auer discloses a freeway access ramp traffic control device which senses lane occupancy on a freeway, exit ramp and access ramp. A conventional red-green traffic signal controls vehicles entering the access ramp in response to congestion on the freeway in order to "meter" the flow of merging traffic onto the freeway. The system does not provide for informing the freeway drivers of the vehicles merging off the access ramp which could cause traffic slowdown and create the potential for collisions.
Another example of a traffic control system for junctions is disclosed in U.S. Pat. No. 3,593,262 granted to Spencer. An access ramp is lined with indicators to create a moving gap between vehicles on the access ramp to match available gaps in the freeway traffic. Again, the freeway drivers are not informed as to the volume or frequency of the merging traffic.
Some traffic control systems have used changeable signs to guide traffic from congested or obstructed roadways. One such system is disclosed in U.S. Pat. No. 3,275,984 granted to Barker. Motion detectors sense the slowdown of freeway traffic and signal the condition to remote signs upstream of the congestion. The signs are not dynamic and have only an on or off condition. They signal avoidance of a roadway rather than the avoidance of collision between individual vehicles merging or exiting the freeway.